Vertical stacking of field effect transistor structures for logic gates

ABSTRACT

Vertically stacked Field Effect Transistors (FETs) are created where a first FET and a second FET are controllable independently. The vertically stacked FETs may be connected in series or in parallel, thereby suitable for use as a portion of a NAND circuit or a NOR circuit. Epitaxial growth over a source and drain of a first FET, and having similar doping to the source and drain of the first FET provide a source and drain of a second FET. An additional epitaxial growth of a type opposite the doping of the source and drain of the first FET provides a body for the second FET.

FIELD OF THE INVENTION

This invention relates generally to Field Effect Transistors (FETs), andmore particularly to vertically stacked FETs suitable for NAND and NORconfiguration.

SUMMARY OF EMBODIMENTS OF THE INVENTION

Semiconductor chips are expensive to manufacture. Therefore, it isimportant to place as much function as possible on a semiconductor chipof a given size. Engineers constantly strive to place logic gates asdensely as possible. Embodiments of the current invention verticallystack Field Effect Transistors (FETs) in order to improve density. Inparticular, embodiments of the invention provide for stacking N-channelField Effect Transistors (NFETs) and for stacking P-channel Field EffectTransistors (PFETs). The NFETs are independently controllable and can beused for an NFET portion of a NAND circuit or a NOR circuit. Likewise,the PFETs are independently controllable and can be used for a PFETportion of a NAND circuit or a NOR circuit. Conventional ComplementaryMetal Oxide Semiconductor (CMOS) logic has NFETs arranged side-by-sideand PFETs also arranged side-by-side.

Vertically stacked FETs are constructed on a semiconductor substrate. Afirst FET on the semiconductor substrate has a first source, a firstdrain, a first gate dielectric, a first body, and a first gateelectrode. A second FET has a second source, a second drain, a secondgate dielectric, a second body, and a second gate electrode. The firstand second gate electrodes are connected to different logical signals.The second gate electrode is physically above the first gate electroderelative to a top surface of the semiconductor substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a vertical structure having a silicon substrate (P-siliconassumed). Alternating layers of dielectric material (HfO₂ used forexemplary purposes) and FET gate conductor (“metal” used for exemplarypurposes) are depicted.

FIG. 2 shows the vertical structure of FIG. 1 in isometric style,further showing how a gate area is formed of the vertical stack. Thegate area includes “dogbone” ends suitable for etching and formingcontacts with the FET gate conductors.

FIG. 3 shows the items in FIG. 2 further covered in silicon dioxide.

FIG. 4 shows the silicon dioxide etched to expose a top of thedielectric (HfO₂). Two contact holes are shown etched for makingcontacts to an FET gate conductor. A cross section AA is identified.

FIG. 5 shows the cross section AA identified in FIG. 4. A dielectric(SiO₂ shown) spacer has been conformally deposited.

FIG. 6 shows the spacer after an anisotropic etch. Source and drainareas have been implanted.

FIG. 7 shows a first growth of an epitaxial layer. The doping of thefirst epitaxial layer is similar to the doping of the source and drainarea (i.e., if the source and drain area are N+, then the firstepitaxial layer is also N+).

FIG. 8 shows an oxygen implant that creates a SiO₂ later isolating asource (or drain) area from the first growth of epitaxial layer. Aphotoresist layer may be used to prevent SiO₂ formation over anothersource (or drain) area, as shown.

FIG. 9 shows a growth of a second epitaxial layer, the second epitaxiallayer doped similarly to the first epitaxial layer. A third epitaxiallayer is grown over the second epitaxial layer, the third epitaxiallayer being of opposite doping to the first and second epitaxial layers(“opposite doping” meaning that if, e.g., the first and second epitaxiallayers are “N” doped, the third epitaxial layer being of opposite dopingwould be “P” doped).

FIG. 10 shows the structure of FIG. 9, planarized.

FIG. 11 shows an etching of a hole through the first and secondepitaxial layers and the SiO₂ later of FIG. 8. This etching will befurther processed to form a lined contact hole.

FIG. 12 shows the hole of FIG. 11 after deposition of a dielectric linerin the lined contact hole.

FIG. 13 shows the lined contact hole filled with a conductive fill. Acontact has been made as shown for making contact to the source or drainarea not contacted by the lined contact hole.

FIG. 14 shows, schematically, two NFETs (N-channel Field EffectTransistors) connected in parallel, suitable for an NFET portion of aNOR circuit.

FIG. 15 shows, schematically, two NFETs connected in series, suitablefor an NFET portion of a NAND circuit.

FIG. 16 shows a completed NAND circuit having parallel-connected PFETs(P-channel Field Effect Transistors) connected in parallel andseries-connected NFETs.

FIG. 17 shows an alternative structure to connect to a source or drainarea using an unlined contact hole if the photoresist of FIG. 8 is notused. Holes are etched to both source and drain areas.

FIG. 18 shows the structure of FIG. 17, with the hole to one of thesource and drain areas being lined with a dielectric.

FIG. 19 shows both holes etched in FIG. 17 being filled with aconductive fill.

FIG. 20 shows a schematic overlaying the structure of FIG. 19 showingtwo NFETs being connected in parallel, suitable for an NFET portion of aNOR gate.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following detailed description of embodiments of the invention,reference is made to the accompanying drawings, which form a parthereof, and within which are shown by way of illustration specificembodiments by which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the invention.

Embodiments of the present invention provide for vertical structures offield effect transistors suitable for NAND and NOR logic gates. Detaileddrawings and description focus on N-channel Field Effect Transistors(NFETs); however, it will be clear that a similar process, withappropriate dopings, will create analogous PFET (P-channel Field EffectTransistors).

With reference now to FIG. 1, a stack 10 comprises a silicon substrate108, shown as being doped P-, forms a substrate for further processingof NFET transistors as will be explained below. It is understood thatPFET transistors will be formed above an N-doped region, for example, anN-well in the silicon substrate 108. Alternating layers of a dielectricmaterial (HfO₂ shown for exemplary purposes) and gate conductor material(e.g., metal or polysilicon; “metal” used for exemplary purposes) arestacked above silicon substrate 108. HfO₂ 101, 102, and 103 are shown inFIG. 1 as the dielectric layers. Metal 105 is layered between HfO₂ 101and HfO₂ 102; Metal 106 is layered between HfO₂ 102 and HfO₂ 103. HfO₂101 and HfO₂ 103 will form gate oxides for a first and a second NFET andtherefore need to be of appropriate thickness for gate oxide purposes.HfO₂ 102 electrically isolates metal 105 from metal 106 and needs to beof appropriate thickness for this purpose.

FIG. 2 shows stack 10 after some processing in a semiconductorfabrication facility to produce a vertical structure 100. An area forNFETs shows a “dog bone” shape. A middle area of the dog bone shape isan area in which NFETs will be created. In the “dog bone”, theorthogonal areas (portions) at the ends are for contacting the gateconductor material. A left orthogonal area shows HfO₂ 101, metal 105,and HfO₂ 102 etched away so that metal 106 can be contacted with dualcontacts. Shapes other than “dog bones” are contemplated for dualcontacts, for example, an “L” shape having a portion long enough to havea dual contact. An “L” having a shorter portion may be used if only asingle contact is allowed in a particular technology. The rightorthogonal area may be used to make contact(s) to metal 105 in a similarmanner. For example the right and left orthogonal areas may be etched atthe same time to remove a portion of HfO₂ 101. In subsequent etches, theleft orthogonal area is further etched, as shown, while the rightorthogonal area is masked to prevent further etching. Note that neitherthe right nor the left orthogonal “dog bone” portion needs to be etchedas shown, nor is a “dog bone” required, if metal 105 or metal 106 isotherwise connected to a source of a logical signal intended to beapplied as a gate voltage on an FET that is created as explained below.For example, metal 105 (or metal 106), during processing in creation ofvertical stack 100, may be routed to such a signal source and thereforea “dog bone” and vias to metal 105 or metal 106 is not required.

FIG. 3 shows the vertical structure 100 after further deposition of SIO₂120, or other suitable dielectric material, to cover vertical structure100. Note that the right orthogonal “dog bone” portion is shown as notetched, whereas the left orthogonal “dog bone” portion has been etched.For example, metal 105 (referenced in FIG. 2) may be routed on the sameconductor level of metal 105 to a source of a signal and therefore notrequire a via.

FIG. 4 shows the vertical structure 100 of FIG. 3 after etching SiO₂ 120until HfO₂ 101 is exposed. Also, holes for gate contacts 125 provide,when filled with conductive material, contacts to metal 106. Holes 121,shown with bold lines, are etched on either side of the remainingvertical structure 100. Holes 121 provide access for subsequentprocessing that will, for example, deposit spacers, etch the spacers,grow epitaxial regions, as will be explained below. FIG. 4 shows crosssection AA which will be used in following figures. Cross section AAcuts through a portion of the remaining vertical structure 100 and holes121 as depicted.

FIG. 5 shows the structure of FIG. 4 at cross section AA, afterconformal deposition of a SiO₂ spacer 130.

FIG. 6 shows the structure of FIG. 5 following an anisotropic etch ofSiO₂ spacer 130. The anisotropic etch bares a top surface of HfO₂ 101and a top surface of P—Si 108. Source/drain regions 132 (132A, 132B) areimplanted into P—Si 108. At this stage of the process, source/drainregions 132 are the source/drains of a first NFET; HfO₂ 103 is a gatedielectric of the first NFET; metal 106 is a gate electrode of the firstNFET.

Source/drain regions 132A and 132B are created by the same implantprocessing step and are generically called source/drain regions 132.However, for clarity as to which source/drain region is intended, asuffix “A” is appended to 132 for the “right hand” (in the drawing)source/drain region 132, and a suffix “B” is appended to 132 for the“left hand” source/drain region 132. A similar convention is usedhereinafter to designate “left hand” and “right hand” portions of aparticular element.

FIG. 7 shows the structure of FIG. 6 with addition of N+ Epi 133 grownover source/drain regions 132. Note the “right hand” and “left hand”“A”, “B” suffix convention. N+ Epi 133 has a doping similar to doping ofsource/drain regions 132. That is, if source/drain regions 132 are doped“N”, N+ Epi 133 is also doped “N”, with appropriate concentration ofdopants.

While detail is given herein for creation of NFETs, it will beunderstood that PFETs may be created in a similar manner, for examplestarting with an N-Nwell in P—Si 108, P+ implantation formingsource/drain regions for a PFET, and P+ epitaxial growth of thesource/drain regions of the PFET.

FIG. 8 shows the structure of FIG. 7 with addition of photoresist 134and an oxygen implant of suitable energy to create SiO₂ 135A oversource/drain region 132A. Photoresist 134 blocks the oxygen implant fromforming a SiO₂ 1358 over source/drain region 1328, as shown in FIG. 7.SiO₂ 135A electrically isolates source/drain region 132A from anoverlying N+ epi 133A. Source/drain region 1328 remains in electricalconnection with similarly doped overlying N+ epi 1338.

FIG. 9 shows the structure of FIG. 8 with addition of continued growthof suitably doped epitaxial silicon, N+ epi 136, shown as N+ epi 136A,1368. N+ epi 136 is grown over N+ epi 133 until N+ epi 136 grows abovethe top surface of HfO₂ 101. N+ epi 136 will “bulge” slightly over thespacer and a portion of HfO₂ 101, as shown. P− epi 137 is grown on N+epi 136, as depicted. P− epi 137 is grown until the top surface of HfO₂101 is covered to a suitable depth for a body of a second NFET. The P−epi 137 is of opposite doping to the N+ doping of N+ epi 136, whereopposite doping means “P” doping versus “N” doping, with appropriateconcentration of dopants for the intended purpose.

N+ epi 133 and N+ epi 136 may be considered a single epitaxial layer.The two-part growth facilitates the oxygen implant to form SiO₂ 135A.

FIG. 10 shows the structure of FIG. 9, following planarization. Theplanarization removes P− epi 137 except for an area above the topsurface of HfO₂ 101. The remaining P− epi 137 forms a body of a secondNFET; HfO₂ 101 forms a gate dielectric of the second NFET. N+ epi 136forms source/drain regions N+ epi 136A and 1368 for the second NFET;metal 105 forms a gate electrode of the second NFET.

FIG. 11 shows the structure of FIG. 10 following etching of a linedcontact hole 150. Lined contact hole 150 is formed by an etch through N+epi 136A and N+ epi 133A, followed by a second etch through SiO₂ 135A.

FIG. 12 shows the structure of FIG. 11 following deposition of adielectric material lining around the vertical surfaces of lined contacthole 150. The dielectric material lining is shown as SiO₂ liner 151.SiO₂ liner 151 may, in embodiments, use a dielectric other than SiO₂, solong as the dielectric is compatible with the processing steps describedherein. Deposition of SiO₂ liner 151 will also form SiO₂ on source/drainregion 132A, and that SiO₂ is removed by etching so that source/drainregion 132A is exposed under lined contact hole 150.

FIG. 13 shows the structure of FIG. 12 following addition of conductivefill 153 in lined contact hole 150. SiO₂ 135A and SiO₂ liner 151electrically isolates the source/drain region 132A from the N+ epi 133Aand N+ epi 136A. Contact 154 is added, as shown, on N+ epi 136B. Contact154 is effectively connected to source/drain region 132B through N+ epi133B and N+ epi 136B. In FIG. 13, a contact 149 is made to the N+ epi136A, and, in an embodiment, contact 149 may be placed as shown, thatis, closer to the second NFET than is the lined contact hole 150.However, in other embodiments, one or more lined contact holes 150 maybe alternated with one or more contacts 149 as shown in the “top view”in FIG. 13, wherein the one or more lined contact holes 150 areapproximately lined up with contacts 149 in order to make the layoutmore compact.

FIG. 14 shows the structure of FIG. 13, including a schematic of a firstNFET N1 and a second NFET N2 overlaid on the structure of FIG. 13.Contact 149 is shown not “lined up” with the lined contact hole 150 inorder to more clearly and completely show connections. Sources of N1 andN2 are shown connected to ground; drains of N1 and N2 are shownconnected together at node output 155. Gate controls of N1 (metal 106)and N2 (metal 105) are independent, assuming that metal 106 and metal105 are connected to independent logical sources. In FIG. 14, N1 and N2are connected as NFETs are in a logical NOR configuration; that is, ifeither N1 or N2 is “on”, output 155 will be pulled to Gnd.

FIG. 15 shows N1 and N2 connected in series, as NFETs are connected in aNAND configuration. Output 155 will be pulled to Gnd if logical signalson both metal 105 and metal 106 are at high logical levels (e.g., Vdd)so that both N1 and N2 are “on”.

FIG. 16 shows a completed NAND, with PFETs P1 and P2 having sourcesconnected to Vdd and drains connected to output 155. NFETs N1 and N2 areconnected as shown in FIG. 15; that is, N1 and N2 are connected inseries between output 155 and Gnd. PFETs P1 and P2 are created, asdescribed earlier, in a manner similar to that used in creation of N1and N1, but built over an N-well N—Si 208, with PFET source/drainregions 232 doped P+ further designated 232A and 232B implanted in N—Si208. Again, note that suffix “A” is appended to “right hand” portions ofthe PFET structure; “B” is added to the “left hand” portions so thatthose portions can be clearly identified when needed. PFET processing,similar to the detailed NFET processing creates a P+ epi 233, a P+ epi236, and an N− epi 238. Appropriate interconnections of gate electrodesare created on metal 105 and metal 106 (or, through vias, otherconducting levels) to provide logical values on the gates of N1, N2, P1,and P2. For example, Metal 105 of N2 is connected to metal 105 of P2;metal 106 of N1 is connected to metal 106 of P1. The connections may bedone on those metal levels (i.e., metal 105 and metal 106) or throughvias such as gate contacts 125 shown in FIG. 4 to other wiring levelssuitable for circuit interconnect from the metal 105 and 106 gateelectrodes of N1, N2, P1, and P2. Whereas FIG. 16 explicitly depicts aNAND, a NOR configuration may be configured by connecting N1 and N2 asshown in FIG. 14, and having P1 and P2 connected in series between Vddand output 155 in a manner similar to the series N1 and N2 shown in FIG.15.

Whereas N1 has been shown earlier as having a lined contact hole 150 toconnect to the source/drain region 132A, and relying on a low impedancepath through similarly doped silicon areas (N+ 1328, N+ Epi 133B, and N+Epi 136B) to connect source/drain region 132B to contact 154 (FIG. 13),in another embodiment, shown in FIGS. 17-20, photoresist 134 (FIG. 8) isnot used, and therefore the oxygen implant creates a SiO₂ 135 (135A,135B) barrier above both source/drain regions 132A and 132B. FIG. 17shows both lined contact hole 150 and unlined contact hole 156 beingcreated at the same time using a silicon etch to etch through N+ epi136A and 136B and N+ epi 133A and 133B, followed by an oxide etchthrough SiO₂ 135A and 135B. In FIG. 18, SiO₂ liner 151 is created inlined contact hole 150, but a mask over unlined contact hole 156prevents creation of a similar SiO₂ liner 151 being formed in unlinedcontact hole 156. FIG. 19 shows both lined contact hole 150 and unlinedcontact hole 156 being filled with a conductive fill 153. Conductivefill 153 may be a metal, such as tungsten, or a suitably dopedpolysilicon fill. A polysilicon fill would typically make a higherresistance contact than a metal fill. Note that one or more contacts 149“in-line” (see top view portion of FIG. 13) is assumed in FIG. 19, sothat N+ epi 136A can be contacted, but do not appear in FIG. 19 becausethey would be “behind” the lined contact hole 150. In an alternativeembodiment, a second lined contact hole may be used in place of theunlined contact hole 156 shown in FIG. 17 and carried through FIGS.17-20; however, one or more contacts such as contact 149 shown in FIG.20 would be required to contact a drain of N2 (i.e., N+ Epi 1368) tooutput 155.

FIG. 20 shows the structure of FIG. 19 overlaid with a schematic of N1and N2 connected in a NOR configuration; that is, if either N1 or N2 isturned on (i.e., if either the voltage on gate electrode metal 105 (forN2) is high, or the voltage on gate electrode metal 106 (for N1) ishigh) output 155 is pulled to Gnd. PFETs to complete the NOR would be asin FIG. 16, with one or more unlined vias used to contact 232B. If alined via is used in the PFET structure, a contact would also need to bemade to P+ Epi 236B.

1. A method for making independently controllable vertically stackedField Effect Transistors (FETs) comprising: forming, on a semiconductorsubstrate, a vertical structure comprising a first gate dielectriclayer, a first conductor layer, a second dielectric layer, a secondconductor layer, and a third dielectric layer; implanting a firstsource/drain region having a first doping type, thereby creating a firstsource and a first drain for a first FET, the semiconductor substrateforming a first body for the first FET; growing a first epitaxial layerhaving a second doping of the same type as the first doping type,thereby forming a second source and a second drain for a second FET;growing a second epitaxial layer having a third doping of opposite typeas the first doping type, thereby forming a second body for the secondFET; creating an isolation between the first source and the overlyingfirst epitaxial layer; connecting the first conductor layer to a firstsignal source; connecting the second conductor layer to a second signalsource.
 2. The method of claim 1 further comprising connecting the firstFET and the second FET in parallel.
 3. The method of claim 1 furthercomprising connecting the first FET and the second FET in series.